Interpretive Summary: Many plant species can maintain additional chromosomes that are not representative of the normal standard chromosome set. These extra chromosomes are called accessory or B-chromosomes. They are usually small and unnecessary for the plants normal development. Accessory chromosomes typically exhibit an accumulation mechanism whereby they can increase in number, often causing deleterious effects to the plant. These small, extra chromosomes have been the focus of various studies for over 60 years due to their unique transmission abilities and unusual morphological features. Italian ryegrass has occasionally been identified to possess accessory chromosomes; however, no detailed study has characterized the behavior of these unusual entities. This study for the first time characterizes the behavior of accessory chromosomes in Italian ryegrass. Cytogenetic evaluations and chromosome counts indicate that this accessory chromosome exhibits the standard survival tendency to maintain itself within the ryegrass population; however, it does not exhibit a high degree of accumulation. This low degree of accumulation allows this material to maintain a relatively low number of accessory chromosomes each generation. This low level of accumulation suggests that this particular accessory chromosome may be a useful vehicle for generating low dose B-ryegrass translocations, B-fescue translocations, or utilization as a platform for foreign gene introduction and maintenance.

Technical Abstract:
Accessory chromosomes, also called B-chromosomes, have been reported in more than 1300 species of plants. Accessory chromosomes show numerical polymorphism and when present in high number, are known to negatively affect the growth and vigor of the plants, while in low numbers they may be beneficial. Accessory chromosomes also exhibit the unusual ability to maintain themselves through a nondisjunction, accumulation mechanism during meiosis. Though accessory chromosomes are known to occur in Lolium, no detailed analysis has been performed. Recently, an Italian ryegrass cultivar possessing accessory chromosomes was released that maintains a low number of accessory chromosomes. This material was utilized as the basis of this study to quantify and qualify the behavior of a Lolium accessory chromosome. Results of the investigation identified that the accessory chromosome of this Lolium cultivar did not exhibit a strong accumulation mechanism and its behavior was relatively normal during meiosis and pollen mitosis. Accessory chromosome transmission through the egg was normal without accumulation and frequent non-disjunction events did not occur during the microspore or pollen development stages. The low levels of nondisjunction behavior could not account for the maintenance of the accessory chromosome in this germplasm. Instead, preferential fertilization of sperm nuclei possessing a B-chromosome are suggested as the maintenance mechanism. Since the Lolium accessory chromosome meiotic and pollen mitotic behavior are relatively normal, exhibiting a low rate of accumulation, such a chromosome may serve as a mini-chromosome carrier platform for directed biolistic-mediated gene transformation and, or the generation and maintenance of accessory chromosome-Lolium and –Festuca translocations.